1. Field of the Invention
The present invention relates to a secondary battery, and more particularly to a can type lithium ion secondary battery having a uniform thickness.
2. Description of the Prior Art
Recently, secondary batteries have actively been studied and developed since they are rechargeable and can be fabricated in smaller sizes with larger capacity. Such secondary batteries include nickel-hydrogen (Ni-MH) batteries, lithium (Li) batteries and lithium ion (Li-ion) batteries.
According to a typical method of fabricating a bare cell of a secondary battery, an electrode assembly including a positive electrode, a negative electrode and a separator is seated in a can generally made from aluminum or an aluminum alloy, the can is subjected to finishing to form a can assembly, an electrolyte is injected into the can, and the can assembly is finally sealed. The can may be usually made from steel. However, a can made from aluminum or an aluminum alloy reduces the weight of an entire battery, due to the small weight of aluminum or an aluminum alloy. Moreover, a can made from aluminum or an aluminum alloy does not corrode even when it is used at high voltages for a long period of time.
As an energy source, batteries may release a great quantity of energy. Especially, secondary batteries not only contain a high density of accumulated energy but also may receive and additionally accumulate energy supplied from another energy source. When an internal short circuit or any other problem occurs in the secondary batteries in such a highly energy-accumulated state or while being recharged, the accumulated energy may be instantly released, thereby causing ignition or explosion of the batteries.
Lithium-based secondary batteries widely used in recent times have a potential fire or explosion hazard because of the high activity of lithium. A lithium ion battery is more stable than a lithium battery because the former use only lithium in ion state (Li+), rather than metallic lithium. However, when the batteries have any internal problems, materials used for a negative electrode or a non-aqueous electrolyte will likely cause significant hazards of fire and explosion due to their combustibility.
Thus, a variety of safety devices are used to prevent the hazards of fire and explosion caused by the internal problems of the batteries being charged or having been charged. Such safety devices are connected to positive and negative terminals of a bare cell by a conductive structure called a “lead plate.” The safety devices can prevent dangerous conditions, such as overheat and fire, by breaking the current when a battery is heated up or charged or discharged beyond its safe limits, resulting in rapid increase of voltage. Safety devices that can be connected to a bare cell include a protective circuit module for detecting any abnormal current or voltage to block the flow of current, a PTC (positive temperature coefficient) thermistor for detecting overheat caused by an abnormal current, and a bimetal strip.
Secondary batteries are fabricated by connecting a circuit section including a safety device to a bare cell by means of a molding resin, covering the lower part of the bare cell with a lower cover and labeling the outer surface of the cell.
FIG. 1 is a perspective view showing the assembly of a lower cover into a conventional can type lithium ion secondary battery. FIG. 2a is a bottom view of the can type lithium ion secondary battery in FIG. 1 wrapped with a label. FIG. 2b is a plan view of the can type lithium ion secondary battery in FIG. 1 with a label.
Referring to FIGS. 1 and 2a and 2b, the conventional can type lithium ion secondary battery includes a bare cell 10 and a circuit molding section 12 with a safety means, such as a protective circuit module, filled by resin on top of the bare cell 10. A molding resin that fills in the circuit molding section 12 may cover even the outer surface of a protective circuit module, while allowing external input and output terminals 16 and 17 to be exposed outside. The circuit molding section 12 can be formed to extend from every side of the bare cell 10 to a predetermined height. Also, a lower cover 20 made of an insulating material, such as a polymer, is connected to the bottom of the bare cell 10. The lower cover 20 is in a box shape with an opened top into which the bare cell 10 can be inserted, with its lower part being in close contact with every inner surface of the lower cover 20. If needed, a separate upper cover 25 can be provided to protect the circuit molding section 12. Certain conventional secondary batteries include no protective circuit module on top of the bare cell 10. Such batteries may provide only the upper cover 25 on top of the bare cell 10, without forming the circuit molding section 12.
As explained above, the exterior can of the bare cell 10 is made of a metal, such as aluminum, and connected to a positive electrode of an electrode assembly provided in the cell 10. The lower cover not only protects but also insulates the bottom of the can.
The bare cell 10 with the lower cover 20 connected is wrapped with a vinyl or polymer film which protects the outer surface of the bare cell 10, serves as a label for entering product information, and electrically insulates the cell 10 from outside.
Due to the tendency to manufacture electronic products in compact sizes, there is a growing demand for smaller and thinner secondary batteries that can be inserted in the compact electronic products. Generally, secondary batteries have a thickness of about 4.7 mm. However, a significant problem experienced by conventional secondary batteries is that they may swell to have a thicker center while being used.
Another problem is that the label overlap increases the thickness of the battery.
For example, as shown in FIG. 2a, the bare cell 10 with the lower cover 20 is wrapped with a label 30 in form of a film. Two opposing ends of the label 30 overlap and are adhered to each other on one side of the bare cell 10, thereby forming a label overlap portion 32. The secondary battery is thicker at the label overlap portion 32 on the lower cover 20 by a thickness of the label 30 (generally about 0.05 mm) than any other portion. The label overlap portion 32 increases the overall thickness of the secondary battery. If the circuit molding section 12 is also wrapped with the label 30, the secondary battery will also be thicker at the label overlap portion 32 on the circuit molding section 12 as shown in FIG. 2b. 